Acetabularia

Acetabularia 

a genus of green algae found in tropical and subtropical seas. The genus includes 21 species. An adult organism of this genus consists of a lobed, rhizoidal base, a cylindrical stalk, and a cap formed by a verticil of connate and unconnate game tangia. In the rhizoidal portion of the single-celled thallus there is one giant nucleus with a developed nucleolus. After the cap is almost completely formed, the nucleus divides repeatedly, and numerous nuclei migrate through the stalk into the gametangia, which are separated by septa. The contents of the gametangia divide into numerous thick-walled cysts, in which meiosis then occurs. In the autumn, after the decomposition of the thallus, the cysts are released and then hibernate; in the spring they germinate, forming biflagellate gametes. The gametes copulate, forming a zygote which during the first year grows into a branched rhizoid with a vertical axis. The latter dies, while the remaining basal portion which contains the nucleus hibernates; the next spring it starts a new vertical axis, which forms one or several sterile verticils on the top. Each verticil of gametangia is preceded by one sterile verticil which drops off. Under laboratory conditions, the life cycle of Acetabularia can be reduced to six months. Acetabularia is a favorite research subject in investigating the interaction between the nucleus and protoplasm.

Kursanov, L. I., and N. A. Komarnitskii. Kurs nizshikh rastenii, 3rd ed. Moscow, 1945. Pages 84–86.

N. P. GORBUNOVA